It is well known (see, e.g., [1]}) that the basis functions best performing in this sense are the ones obtained through a Proper Orthogonal Decomposition (POD) applied to the snapshots matrix \(\mathbf {S}\) . The eigenproblem
\(\mathbf {C} \mathbf {V} = \mathbf {V} \mathbf {\lambda },\) 
